in a healthy person the function of breathing is entirely spontaneous. The brain senses a build-up of carbon dioxide in the blood and immediately calls for more oxygen. This oxygen is taken into the body by spontaneous inspiration and carbon dioxide is removed in the passive exhalation phase of respiration. A healthy person generates a certain amount of humidity, which is used in the lung to stop the build-up of secretions.
The ability to breathe spontaneously may be lost for a number of reasons. Examples are as a result of surgical procedures (post-operatively), as a result of certain muscular disorders affecting the lung, or as a result of sedation by a clinician. Patients thus affected must be ventilated by mechanical means in order to achieve oxygenation and carbon dioxide removal.
When a patient is mechanically ventilated it is essential that the humidity of the air is maintained at a sufficiently high level, since a lung with impaired function will be more susceptible to secretions. This can be achieved using a heat-moisture exchanger (HME) or a heated water bath humidifier. An HME retains the moisture in an exhaled breath and this moisture is sent back to the lung with the next inspiratory phase. In a water bath system the inspiratory gas is passed through a heated water chamber and picks up humidity prior to entering the lung. As the humid inspiratory gas travels along the breathing system a certain amount of water vapour will cool and start to condense, forming water droplets, which will start to build up, causing so-called “rain-out”.
Such water has to be removed from the breathing system so that it does not occlude the respiratory air flow or drain back into the patient's lungs thereby putting the patient at risk of drowning, or does not drain into the ventilator/anaesthetic equipment thus causing damage. If it is allowed to accumulate for a protracted period then due to its non-compressible nature the water will effectively block the breathing system.
The most effective way of collecting moisture in such a system is by the use of a device called a water trap. Such a device is generally located at the mid-point of the breathing system and positioned at the lowest point so that liquid will drain into it. It is necessary periodically to empty accumulated water from such a water trap and this should ideally be possible without interrupting ventilation of the patient and also without causing a drop in the pressure within the system, and without permitting escape of possibly contaminated or infectious fluid from the system or infection of the system by external agents.
EP 0705616 discloses one solution to this problem which utilises a two disc valve arrangement with each disc including a semi-circular aperture. The valve is opened and closed by the apertures being brought into or out of registration respectively. WO 01/78819 discloses another solution which also utilises a two disc valve arrangement. This arrangement however uses two apertures in each disc surrounded by elastomeric seals to improve the sealing ability of the valve. The construction of these water traps is relatively complicated with each water trap comprising several separate components.